This application is based on Patent Application No. 11-236449 (1999) filed Aug. 24, 1999 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a suction recovery method and an ink jet printing apparatus. The present invention is applicable to general printing apparatuses, apparatuses such as copiers, facsimile terminal equipment having a communication system, and word processors having a printing section, as well as industrial printing apparatuses combined with various processing apparatuses in a compound manner.
2. Description of the Prior Art
In ink jet printers, when volatile components of an ink evaporate from a tip of a nozzle of an ink jet print head, the ink becomes more viscous, the concentration of an ink dye increases, or the ink is fixed. In addition, when the ink is left in the print head, bubbles may occur in a liquid chamber of the print head. The bubbles prevent the ink from being normally supplied, and in the worst case, the ink is totally precluded from flowing through the print head, thereby seriously affecting printing operations.
To avoid this problem, a suction recovery method is widely used which caps a face of the print head while using a pump previously provided in the printer to reduce pressure inside the cap in order to draw out the ink from the nozzle.
Pumps used for this suction recovery can be roughly classified into piston pumps and tube pumps.
A method with a piston pump comprises moving a piston of the piston pump to reduce the pressure of a vacuum chamber 10 in communication with the cap and then allowing the vacuum chamber 10 and the interior of the cap to communicate with each other, as shown in FIG. 25. As a result, a negative pressure is generated inside the cap as shown in FIG. 26. As is apparent from this figure, the method with the piston pump enables a high negative pressure to be instantaneously applied to the print head. The negative pressure disappears as the ink flows into the cap.
A method with a tube pump comprises generating a negative pressure inside the cap using a recovery force of a tube 21, which has been squeezed by a roller 20, as shown in FIG. 27. The tube pump enables the amount of suction to be arbitrarily set by varying the amount of squeezing and also enables suction pressure to be arbitrarily set by varying squeezing speed. This method, however, increases the negative pressure while sucking the ink, the ink is wasted until a desired suction pressure is reached. In addition, to increase the suction pressure quickly, the squeezing speed must be augmented, thereby requiring a strong driving force.
FIG. 28 shows temporal variations in negative pressure observed when the tube pump is used for suction. This figure shows that in an area A, the suction pressure (negative pressure) increases consistently with time, whereas in an area B, the balance is held between the negative pressure generated by the pump and cancellation of the negative pressure effected by the drawn-out ink, resulting in a balanced state. The negative pressure in the balanced state (a saturated pressure) depends on flow resistance of the head during the suction and on the capabilities of the pump. Typically, a suction recovery operation is performed in the area A.
Unlike the tube pump, the piston pump does not enable suction conditions (the suction pressure and the amount of suction) to be arbitrarily set by means of control during operation. Additionally, due to the instantaneously high negative pressure, ink flow speed may increase excessively during suction. If the ink is likely to generate a large amount of bubbles, the method with the piston pump causes bubbles to be generated in the head during the suction, resulting in a trouble.
Consequently, if the suction conditions are to be set precisely or the printer uses an ink that is likely to generate a large amount of bubbles, the tube pump is often used.
Precisely speaking, the suction recovery performance should be set using the ink flow speed and the flow rate measured at this speed. The ink flow speed, however, varies over time, and it is thus difficult to measure this speed to quantitatively set the suction recovery performance. As a result, the suction recovery performance is typically controlled based on the suction pressure and the amount of suction.
The method with the tube pump, however, has problems originating from a continuous increase in suction pressure during the ink suction.
That is, since the amount of suction and the suction pressure cannot be set independently, the ink may be wasted and bubbles may be mixed in the liquid chamber.
For example, in a system where the suction pressure is likely to rise, that is, a system where the suction pressure increases as shown by a steep curve, the suction pressure rises excessively in order to maintain the amount of suction. The excessive increase in suction pressure causes an increase in ink flow speed. The ink supply capacity of an ink tank per unit time has an upper limit, so that at a flow rate higher than this upper limit, the ink is inappropriately supplied to cause bubbles to be mixed in the liquid chamber.
On the other hand, in a system where the suction pressure is unlikely to rise, that is, a system where the suction pressure increases as shown by a gentle curve, an excess amount of ink must be sucked in order to maintain the suction pressure. This causes the ink to be wasted.
To avoid these problems, the tube pump configuration must be determined in such a manner that the suction pressure and the amount of suction correspond mutually. This, however, lessens the advantage of the tube pump of being able to arbitrarily determine recovery conditions.
Furthermore, for an ink jet apparatus having a function for using different recovery modes with step-by-step varying recovery performances for different situations, conditions being met for each of the recovery modes must be determined. It is thus very difficult to design such an ink jet apparatus.
The present invention is provided in view of these circumstances, and it is an object thereof to provide a suction recovery method and apparatus for ink jet printing apparatuses that can maintain, during suction recovery operations, a suction recovery performance while avoiding wasting inks and mixing bubbles in a liquid chamber.
To solve the above described problems, the present invention provides a suction recovery method for an ink jet printing apparatus for driving a tube pump connected to a cap while a face of a print head is capped by the cap, to generate a negative pressure inside the cap to suck an ink from the print head, the method being characterized by comprising a first step of continuously driving the tube pump to set an interior of the cap at a target negative pressure and a second step of stopping the tube pump and then driving it again.
In the second step, when a preset period of time has passed since the continuous driving of the tub pump was started, the interior of the cap can be exposed to the atmosphere. Alternatively, in the second step, stopping and driving of the tube pump can be repeated a number of times to carry out idle suction even after the interior of the cap has been exposed to the atmosphere. Alternatively, this method can further comprise a third step operating after the second step to stop the tube pump and then drive it again.
That is, according to the present invention, by repeating driving and stopping the pump during the suction, a predetermined suction pressure is maintained to enable a suction operation without excessively increasing the suction pressure. This is particularly effective on print heads with small ejection ports.
Thus, according to the present invention, the tube pump is continuously rotated to quickly set the interior of the cap at the target negative pressure, and the driving and stopping of the tube pump is then repeated a number of times to maintain the interior of the cap within a predetermined range near the target negative pressure. Consequently, the suction can be recovered with an appropriate amount of suction and an appropriate suction pressure for the print head to restrain the wasteful use of inks while preventing bubbles from being sucked. As a result, suction can be reliably recovered.
Alternatively, the present invention provides a suction recovery method for an ink jet printing apparatus for driving a tube pump connected to a cap while a face of a print head is capped by the cap, to generate a negative pressure inside the cap to suck an ink from the print head, the method being characterized by comprising a first step of continuously driving the tube pump at a first predetermined speed to set an interior of the cap at a target negative pressure and a second step of driving the tube pump at a second speed lower than the first speed.
In the second step, when a preset predetermined period of time has passed since the continuous driving of the tub pump was started, the interior of the cap can be exposed to the atmosphere. Alternatively, in the second step, the tube pump can be driven to carry out idle suction even after the interior of the cap has been exposed to the atmosphere.
Thus, according to the present invention, the tube pump is continuously rotated at a predetermined driving speed to quickly set the interior of the cap at the target negative pressure, and is then driven at a reduced driving speed to maintain the target negative pressure. Consequently, the suction pressure remains at the same level, and the suction can be recovered with an appropriate amount of suction and an appropriate suction pressure for the print head to restrain the wasteful use of inks while preventing bubbles from being sucked.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.